Process for producing porous polyimide resin and porous polymide resin

ABSTRACT

A method of producing porous polyimide resin that enables pores to be formed in a precursor of polyimide resin, with its form of microphase-separated structure wherein a dispersive compound is dispersed in the precursor of polyimide resin being kept unchanged, so as to provide significantly reduced dielectric constant and also provide improvement in mechanical strength and heat resistance, and the porous polyimide resin produced in the same producing method. A coating comprising porous polyimide resin is formed by applying resin solution comprising a precursor of polyimide resin and a dispersive compound and then drying a solvent, to form a coating in which the dispersive compound is dispersed in the precursor of polyimide resin; extracting the dispersive compound from the coating for removal to make the precursor of the polyimide resin porous; and imidizing the coating after preheated in a temperature range of 190-250° C.

TECHNICAL FIELD

[0001] The present invention relates to a method of producing porouspolyimide resin and to porous polyimide resin. More particularly, thepresent invention relates to a method of producing porous polyimideresin suitably used as insulating material for electronic/electricequipment and electronic components and to porous polyimide resinproduced by the same producing method.

BACKGROUND ART

[0002] In general, polyimide resin has a high insulation performance.Accordingly, the polyimide resin is widely applied toelectronic/electric equipment and electronic components includingcircuit boards and printed wiring boards as parts or componentsrequiring high reliability.

[0003] In recent years, the electronic equipment is increasinglydemanded to store large amounts of information corresponding to thehighly-networked information society, process them rapidly, and transmitthem at high speed. Accordingly, the polyimide resin used in theelectronic equipment is also demanded to have higher performance,particularly reduction in dielectric constant and reduction indielectric loss tangent, for electric properties corresponding to higherfrequencies.

[0004] An attempt to provide reduction in dielectric constant ofpolyimide resin was proposed, for example, by Japanese Laid-open(Unexamined) Patent Publication No. 2000-44719, according to which afterhydrophilic polymer is dispersed in a precursor of polyimide resinsoluble in an organic solvent, the hydrophilic polymer is removed bybaking it or by extraction of the solvent, to form pores in theprecursor of polyimide resin to thereby produce porous polyimide resin.

[0005] When the pores are formed in the precursor of polyimide resin byremoving the hydrophilic polymer, it is ideal that the form of themicrophase-separated structure wherein the hydrophilic polymer isdispersed in the precursor of polyimide resin is kept unchanged.However, when the pores are formed in the precursor of polyimide resinby removing hydrophilic polymer by baking it or by extraction of thesolvent, followed by imidization of the precursor of polyimide resin,the pores are flattened or clogged, so that the porosity is reducedbelow the ideal one, resulting in a problem that sufficient reduction ofthe dielectric constant cannot be obtained.

[0006] Although it is preferable that the pores has a smaller diameterto retain mechanical strength and heat resistance of the porouspolyimide resin produced, when the pores are reduced to a diameter assmall as 1 μm or less, there arises the specific problem that the poresmay be clogged with ease.

DISCLOSURE OF THE INVENTION

[0007] It is the object of the invention to provide a method ofproducing porous polyimide resin that enables pores to be formed in theprecursor of polyimide resin, with the form of the microphase-separatedstructure wherein a dispersive compound is dispersed in the precursor ofpolyimide resin being kept unchanged, so as to provide sufficientlyreduced dielectric constant and also provide good mechanical strengthand heat resistance, and the porous polyimide resin produced by the sameproducing method.

[0008] The present invention provides a method of producing porouspolyimide resin comprising the resin solution preparing process ofpreparing resin solution comprising a precursor of polyimide resin, adispersive compound capable of dispersing in the precursor of polyimideresin, and a solvent; the coating forming process of forming a coatingin which the dispersive compound is dispersed in the precursor ofpolyimide resin by application of the resin solution, followed by dryingof the solvent; the extracting and removing process of extracting thedispersive compound from the coating for removal by use of extractingsolvent; the preheating process of preheating the coating in atemperature range of 190-250° C.; and the imidizing process of imidizingthe coating.

[0009] In this producing method, the coating is preferably heated in atemperature range of 270-400° C. in the imidizing process. Also, it ispreferable that the dispersive compound is dispersed in the precursor ofpolyimide resin, with its average particle size of not more than 1 μm,in the coating forming process.

[0010] In this producing method, it is preferable that the dispersivecompound has a weight-average molecular weight of not more than 10,000.Also, it is preferable that the extracting solvent used in theextracting and removing process is liquefied carbon dioxide or carbondioxide that is in its super critical state.

[0011] In addition, the present invention includes a porous polyimideresin which is produced in a producing method comprising the resinsolution preparing process of preparing resin solution comprising aprecursor of polyimide resin, a dispersive compound capable ofdispersing in the precursor of polyimide resin, and a solvent; thecoating forming process of forming a coating in which the dispersivecompound is dispersed in the precursor of polyimide resin by applicationof the resin solution, followed by drying of the solvent; the extractingand removing process of extracting the dispersive compound from thecoating for removal by use of extracting solvent; the preheating processof preheating the coating in a temperature range of 190-250° C.; and theimidizing process of imidizing the coating, and which includes poreshaving an average pore size of not more than 1 μm. Further, the porouspolyimide resin preferably has a dielectric constant of not more than 3.

[0012] According to the porous polyimide resin producing method of thepresent invention, since the form of the pores (cells) in the precursorof polyimide resin obtained by extracting the dispersive compound forremoval in the preheating process is retained, the form of themicrophase-separated structure can be held as it is in the nextimidizing process and accordingly the pores can be effectively preventedfrom being flattened or clogged. Due to this, even the similarly sizedmicroscopic pores (cells) having an average pore size of e.g. 1 m orless can be well formed without being clogged or closed and, as a resultof this, the porous polyimide resin having a dielectric constant as lowas not more than 3 or further not more than 2.5 can be obtained.

[0013] Accordingly, since the porous polyimide resin of the presentinvention can provide good high-frequency characteristics, mechanicalstrength, and heat resistance, the porous polyimide resin of the presentinvention can be effectively used as polyimide resins used in theelectronic/electric equipment and the electronic components for storinglarge amounts of information, processing them rapidly, and transmittingthem at high speed, including internal insulating material of theelectronic equipment, cushioning material and an insulating layer of thecircuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 illustrates a production process of an embodiment of aporous polyimide resin producing method of the present invention:

[0015] (a) illustrates the process of applying resin solution to asubstrate;

[0016] (b) illustrates the process of drying the resin solution;

[0017] (c) illustrates the process of extracting a dispersive compoundfrom a precursor of polyimide resin for removal; and

[0018] (d) illustrates the process of forming the porous polyimide resinby imidizing the precursor of polyimide resin,

[0019]FIG. 2 is a photograph taken with a scanning electron microscope,showing a sectional view of a coating comprising porous polyimide resinof Example 1,

[0020]FIG. 3 is a photograph taken with the scanning electronmicroscope, showing a sectional view of a coating comprising porouspolyimide resin of Example 2, and

[0021]FIG. 4 is a photograph taken with the scanning electronmicroscope, showing a sectional view of a coating comprising porouspolyimide resin of Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] In the method of producing porous polyimide resin of the presentinvention, resin solution containing a precursor of polyimide resin, adispersive compound capable of dispersing in the precursor of polyimideresin, and a solvent is prepared in the resin solution preparingprocess, first.

[0023] The precursor of polyimide resin used in the present invention isthe precursor of polyimide resin from which polyimide resin can beformed by imidization. Known precursors, such as polyamic acid resin,can be used without any particular limitation.

[0024] The polyamic acid resin can be produced, for example, by reactingorganic tetracarboxylic dianhydride with diamine. The organictetracarboxylic dianhydrides that may be used include, for example,pyromelletic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride,2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride,2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride,3,3′,4,4′-benzophenonetetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)-ether dianhydride, andbis(3,4-dicarboxyphenyl)-sulfonic dianhydride. Those may be used singlyor in combination of two or more.

[0025] The diamines that may be used include, for example,m-phenylenediamine, p-phenylenediamine, 3,4′-diaminodiphenyl ether,4,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl sulfone,3,3′-diaminodiphenyl sulfone, 2,2-bis(4-aminophenoxyphenyl)propane,2,2-bis (4-aminophenoxyphenyl)hexafluoropropane,1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene,2,4-diaminotoluene, 2,6-diaminotoluene, diaminodiphenylmethane,4,4′-diamino-2,2-dimethylbiphenyl, and2,2-bis(trifluoromethyl)-4,4′-diaminobiphenyl. Those may be used singlyor in combination of two or more.

[0026] The polyamic acid resin can be produced in the form of solutionof polyamic acid resin, for example, by reacting organic tetracarboxylicdianhydride with diamine in such a ratio as to be a substantiallyequimolar ratio in a suitable organic solvent like a polar solvent, suchas N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide,and dimethylsulfoxide, usually at 0-90° C. for 1-24 hours.

[0027] The polyamic acid resin thus produced has a unit structureexpressed by the following general formula (1) and has a weight-averagemolecular weight in the approximate range of 5,000-200,000, orpreferably 10,000-100,000.

[0028] (where R_(a) represents a bivalent organic group and R_(b)represents a quadrivalent organic group).

[0029] In addition to the polyamic acid resin mentioned above, forexample polyamic acid resin having carboxyl groups to be bonded to R_(b)of the general formula (1) given above, either or both of the carboxylgroups being esterified by a compound having hydroxyl group, can also becited as the precursor of polyimide resin. It is preferable that any ofthe precursors of the polyimide resins are prepared in the form ofsolution of reactive solvent.

[0030] Any dispersive compounds that can be mixed and dispersed in theprecursor of polyimide resin and extracted for removal therefrom byusing an extracting solvent, as mentioned later, may be used to thepresent invention without any particular limitation. To be morespecific, the dispersive compounds that may be microphase-separated intothe fine-grained form in the precursor of polyimide resin can preferablybe used. For example, polyethylene glycol, polypropylene glycol andtheir derivatives capped with methyl at half-end or both-end thereof,or, their derivatives capped with (metha)acrylate at half-end orboth-end thereof, urethane prepolymer, phenoxypolyethyleneglycol(metha)acrylate, ε-caprolactone(metha)acrylate, trimethylolpropanetri(metha)acrylate, dipentaerithritol hexa(metha)acrylate,urethane(metha)acrylate, epoxy(metha)acrylate, andoligoester(metha)acrylate can be cited as the dispersive compound. Thosedispersive compounds may be used singly or in combination of two ormore.

[0031] The dispersive compound usually has an average particle diameterof 0.05-3 μm, or preferably 0.05-1 μm. In order to form themicrophase-separated structure having a pore size (cell size) of notmore than 1 μm in the precursor of polyimide resin in the next coatingforming process, the dispersive compound preferably has a weight-averagemolecular weight of not more than 10,000, usually in the range of100-3,000, or preferably in the range of 250-1,500. When theweight-average molecular weight of the dispersive compound is in excessof 10,000, compatibility of the dispersive compound with the precursorof polyimide resin may reduce so that the phase-separated structureformed may increase.

[0032] The amount of dispersive compound mixed is suitably determined,taking the weight-average molecular weight of the dispersive compoundand others into consideration. For example, in order to form themicrophase-separated structure having the cell size of not more than 1μm in the precursor of polyimide resin, not more than 200 parts byweight of dispersive compound per 100 parts by weight of precursor ofpolyimide resin is preferably mixed. Also, in order to provide the voidcontent required for the porous polyimide resin obtained to have thedielectric constant of not more than 3, at least 10 parts by weight ofdispersive compound per 100 parts by weight of precursor of polyimideresin is preferably mixed.

[0033] The reactive solvent used to synthesize the precursor ofpolyimide resin may be used as it is in the present invention. Further,in addition to or in place of the reactive solvent, organic solvent,such as 1,3-dimethyl-2-imidazolidinone, diglyme, triglyme,tetrahydrofuran, dioxane, cyclohexane, toluene, and xylene, may be usedsingly or in combination of two or more.

[0034] Although no particular limitation is imposed on the amount ofsolvent mixed, the amount of solvent mixed is adjusted to adjust theviscosity of resin solution for the intended purpose and application.Usually, the solvent is mixed in a weight ratio of the same quantity to100 times, or preferably 2 times to 50 times as much as the total amountof precursor of polyimide resin and dispersive compound.

[0035] In the resin solution preparing process, the precursor ofpolyimide resin, the dispersive compound and the solvent may be properlymixed in the ratio mentioned above in a known method, to prepare theresin solution. Specifically, when the reactive solvent used tosynthesize the precursor of polyimide resin is used as it is, thedispersive compound may be blended directly in the solvent in which theprecursor of polyimide resin is dissolved in the ratio mentioned above,followed by being mixed therewith under stirring.

[0036] Sequentially, in the coating forming process of the porouspolyimide resin producing method of the present invention, after theapplication of the resin solution, the solvent is dried to form thecoating in which the dispersive compound is dispersed in the precursorof polyimide resin.

[0037] For example, the resin solution may be applied as shown in FIG.1(a). That is to say, a specified substrate 1 is prepared and the resinsolution 2 is applied to the substrate 1.

[0038] The materials that may be used for the substrate 1 include, forexample, foils or sheets of metals or alloys thereof, such as copper,aluminum, stainless, copper-beryllium, phosphor bronze, and iron-nickel,foils or sheets of ceramics, such as silicon wafer, and glass, and filmsof plastics, such as polyimide and polyester.

[0039] The substrate may be coated with the resin solution in anyadequate known coating process, such as a spin coater and a bar coater,in accordance with the form of the substrate and the thickness of thecoating. It is preferable that the resin solution is applied so that thecoating after dried may have thickness of 0.1-50 μm, or preferably 1-25μm.

[0040] Before the application of the resin solution, the first coat ofsilane coupling agent or titanate coupling agent may be given to thesurface of the substrate to provide improvement in adhesion.

[0041] The solvent may be dried by heating at 40-200° C. in usual, orpreferably 60-190° C., though it depends on the types of solvent. Thedrying time is usually in the range of 1-30 minutes, though it dependson the heating temperature. As the result of this drying of the solvent,the dispersive compound 3 is insolubilized in the precursor 4 ofpolyimide resin and is microphase-separated in the form of microscopicparticles and, as a result, the coating 5 in which the dispersivecompound 3 is uniformly dispersed in the precursor 4 of polyimide resinis formed, as model-wisely shown in FIG. 1(b).

[0042] It is preferable that the dispersive compound is dispersed sothat the microphase-separated structure having a pore size (cell size)of not more than 1 μm can be formed in the precursor of polyimide, inorder to provide reduction in dielectric constant, as mentioned above.

[0043] The solvent may be dried at one-time drying or may be dried attwo or more times by changing temperature stepwise.

[0044] In the porous polyimide resin producing method of the presentinvention, the dispersive compound is extracted for removal from thecoating in which the dispersive compound is dispersed in the precursorof polyimide resin by using the extracting solvent in the extracting andremoving process.

[0045] No particular limitation is imposed on the extracting solvent.Organic solvent usually used as the extracting solvent may be used inthe present invention. From the viewpoints of removal efficiency andharmlessness, liquefied carbon dioxide or carbon dioxide that is in itssuper critical state (supercritical carbon dioxide) is preferably used.

[0046] For example, when the superficial carbon dioxide is used, theextracting temperature is just required to be equal to or higher thanthe critical point, as well as in the range in which the precursor ofpolyimide resin is not imidized extremely in the extracting and removingprocess. In practice, since solubility of the dispersive compound in thesupercritical carbon dioxide reduces with increase in the temperature,the extracting temperature is preferably in the range of 32-180° C., orpreferably 40-150° C. Also, the extracting pressure is just required tobe equal to or higher than the critical point. To be more specific, theextracting pressure is preferably in the range of 7.3-100 MPa, orpreferably 10-50 MPa. Further, the extracting time is usually in theapproximate range of 1-30 hours, though it depends on the extractingtemperature, the extracting pressure, or the amount of dispersivecompound mixed which is dispersed in the precursor of polyimide resin.

[0047] No particular limitation is imposed on the extracting method. Forexample, after the critical carbon dioxide is pressurized, it may be fedcontinuously into a pressure vessel containing the coating by using ametering pump. Alternatively, the supercritical carbon dioxidepressurized to a specified pressure may be fed into the pressure vesselcontaining the coating.

[0048] As a result of this, the dispersive compound 3 is extracted andremoved from the precursor 4 of polyimide resin and thereby the porouscoating 5 is formed, as shown in FIG. 1(c).

[0049] Sequentially, in the porous polyimide resin producing method ofthe present invention, the porous coating of the precursor of polyimideresin is preheated in the temperature range of 190-250° C. in thepreheating process.

[0050] The coating may be preheated in a known manner. The preheatingcan allow the precursor of polyimide resin to be imidized to a moderatedegree. Due to this, even when the coating is imidized in the nextimidizing process, the pores (cells) are held (fixed) in their forms, sothat they are prevented from being flattened or clogged in the imidizingprocess. The preheating time is usually in the range of 1-40 hours, orpreferably 2-24 hours, though it depends on the preheating temperature.

[0051] Then, in the porous polyimide resin producing method of thepresent invention, the coating is imidized in the imidizing process toobtain the porous polyimide resin.

[0052] The coating may be imidized by heating in a known manner. Forexample, the coating is usually heated in the temperature range of270-400° C. for a few hours in a vacuum or in an atmosphere of an inertgas. As a result of this, the heating/dehydration/ring-closure reactionof the precursor of polyimide resin is carried out. As a result of this,the coating 5 comprising the precursor 4 of the polyimide resin as wasmade porous is imidized to form a coating 7 comprising the porouspolyimide resin 6, as shown in FIG. 1(d).

[0053] The precursor of polyimide resin as was made porous may beimidized under the action of dehydration cyclization agent, such as themixture of acetic anhydride and pyridine, in the imidizing process.

[0054] According to the porous polyimide resin producing method of thepresent invention, since the form of the pores (cells) in the precursorof polyimide resin obtained by extracting the dispersive compound forremoval in the preheating process is retained, the form of themicrophase-separated structure can be held as it is in the nextimidizing process and accordingly the pores can be effectively preventedfrom being flattened or clogged. Due to this, even the similarly sizedmicroscopic pores (cells) having an average pore size of e.g. 1 μm orless can be well formed without being clogged or closed and, as a resultof this, the porous polyimide resin having a dielectric constant as lowas not more than 3 or further not more than 2.5 can be obtained.

[0055] Accordingly, since the porous polyimide resin of the presentinvention can provide good high-frequency characteristics, mechanicalstrength, and heat resistance, the porous polyimide resin of the presentinvention can be effectively used as polyimide resins used in theelectronic/electric equipment and the electronic components for storinglarge amounts of information, processing them rapidly, and transmittingthem at high speed, including internal insulating material of theelectronic equipment, cushioning material and an insulating layer of thecircuit board.

[0056] In the present invention, known sensitizers, such as derivativeof dihydropyridine, derivative of diazonaphthoquinone ester sulfonate,and aromatic diazide compound, may be blended suitably, for example, inthe resin solution preparing process so that the formed coating may bepatterned in a specific form by exposing it to light for development ina known manner in any desirable process after the coating formingprocess (for example, in the process between the extracting and removingprocess and the imidizing process).

EXAMPLES

[0057] While in the following, the present invention will be describedin further detail with reference to Examples and Comparative Example,the present invention is not limited to any of Examples and ComparativeExample.

Synthesizing Example 1

[0058] After 10.8 g of p-phenylenediamine was poured in a 500 mLseparable flask having an agitator and a temperature gauge, 247.2 g ofN-methyl-2-pyrrolidone (NMP) was added thereto and stirred to dissolvethe p-phenylenediamine.

[0059] Then, 29.4 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride wasgradually added to the solution and was stirred continuously attemperature of not higher than 30° C. for two hours, to obtain thesolution of the precursor of polyimide resin having the concentration of14 weight percent.

Synthesizing Example 2

[0060] After 29.2 g of 1,4-bis(4-aminophenoxy)benzene was poured in a500 mL separable flask having an agitator and a temperature gauge, 360.3g of N-methyl-2-pyrrolidone (NMP) was added thereto and stirred todissolve the 1,4-bis(4-aminophenoxy)benzene.

[0061] Then, 29.4 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride wasgradually added to the solution and was stirred continuously attemperature of not higher than 30° C. for two hours, to obtain thesolution of the precursor of polyimide resin having the concentration of14 weight percent.

Example 1

[0062] 30 parts by weight of the solution of the precursor of polyimideresin obtained in the synthesizing example 2 was mixed with 70 parts byweight of the solution of the precursor of polyimide resin obtained inthe synthesizing example 1, to obtain the mixed solution of theprecursor of polyimide resin. Then, polyethylene glycol dimethyletherhaving the weight-average molecular weight of 500 was blended in themixed solution of the precursor of polyimide resin in the proportion of66 parts by weight per 100 parts by weight of resin contents in themixed solution of the precursor of polyimide resin and was stirred, toobtain transparent homogeneous resin solution.

[0063] Then, the resin solution thus obtained was applied to a stainlessfoil (SUS304) having thickness of 25 μm by using the spin coater so thatthe coating could have thickness of 21 μm after dried. Then, the resinsolution was dried at 95° C. for ten minutes in a hot-air circulationoven to release NMP therefrom. Thereafter, the resin solution was driedfurther at 18° C. for twenty minutes in the hot-air circulation oven tobe completely dried and thereby the coating comprising the precursor ofpolyimide resin having the microphase-separated structure ofpolyethylene glycol dimethylether was formed.

[0064] Sequentially, after the coating thus obtained was cut to a 100mm×60 mm sheet, the sheet was put in the 500 mL pressure vessel. Afteran interior of the pressure vessel was pressurized to 25 MPa in anatmosphere of 100° C., the carbon dioxide was fed in the pressure vesselat a flow rate of about 5 L per minute by gas volume while keeping thatpressure and discharged therefrom, to carry out the operations toextract the polyethylene glycol dimethylether for two hours.

[0065] Sequentially, the coating was preheated at 250° C. for 24 hoursso that the form of the pores (cells) in the precursor of polyimideresin produced by extracting the polyethylene glycol dimethylether forremoval from the precursor of polyimide resin could be retained (fixed).

[0066] Thereafter, the coating was heated at 375° C. for two hours in avacuum of 1.3 Pa, to form the coating comprising the polyimide resin.

[0067] The SEM observation image of a sectional view of the coatingcomprising the porous polyimide resin obtained was made by the imageprocessing. The result is shown in FIG. 2. The size of the pores (cells)calculated by the image processing was 0.193 μm. Also, the dielectricconstant (ε) of the coating comprising the porous polyimide resinobtained was 2.539 (measuring frequency: 1 MHz). The measuring method(the same measuring methods were used in the sequent Example andComparative Example) is described below.

[0068] SEM Observation: The coating film comprising the porous polyimideresin was cut at an accelerating voltage of 30 kV and a beam voltage of0.8 μA by using focused ion beam processing equipment (FIB) (SM19200available from Seiko Instruments, Inc.) and using liquid metal Ga as theion source. The sectional view of the coating was observed with thescanning electron microscope (SEM) (S-570 available from Hitachi, Ltd.)at an accelerating voltage of 10 kV.

[0069] Dielectric Constant: The dielectric constant of the coating wasmeasured with HP4284A Precision LCR Meter available from YokokawaHewlett-Packard Co.

Example 2

[0070] Except that polyethylene glycol dimethylether having theweight-average molecular weight of 500 was blended in the mixed solutionof the precursor of polyimide resin in the proportion of 80 parts byweight per 100 parts by weight of resin contents in the mixed solutionof the precursor of polyimide resin, to prepare the resin solution, thesame operations as the operations of Example 1 were made to form thecoating comprising the porous polyimide resin.

[0071] The SEM observation image of a sectional view of the coatingcomprising the porous polyimide resin obtained was made by the imageprocessing. The result is shown in FIG. 3. The size of the pores (cells)calculated by the image processing was 0.194 μm. Also, the dielectricconstant (ε) of the coating obtained was 2.200 (measuring frequency: 1MHz).

Comparative Example 1

[0072] Except that the coating was not preheated at 250° C. for 24hours, the same operations as the operations of Example 1 were made toform the coating comprising the porous polyimide resin.

[0073] The SEM observation image of a sectional view of the coatingcomprising the porous polyimide resin obtained was made by the imageprocessing. The result is shown in FIG. 4.

[0074] Though the size of the pores (cells) in the coating calculated bythe image processing was 0.199 μm, the pores were clogged, as seen fromFIG. 4. Hence, Comparative Example 1 has a reduced number of pores, ascompared with Examples 1 and 2, for the reason of which the dielectricconstant (ε) of the coating was 3.04 (measuring frequency: 1 MHz).

[0075] While the illustrative embodiments of the present invention areprovided in the above description, such is for illustrative purpose onlyand it is not to be construed restrictively. Modification and variationof the present invention that will be obvious to those skilled in theart is to be covered by the following claims.

Capabilities of Exploitation in Industry

[0076] The porous polyimide resin of the present invention produced inthe porous polyimide resin producing method of the present invention canbe effectively used as polyimide resins used in the electronic/electricequipment and the electronic components for storing large amounts ofinformation, processing them rapidly, and transmitting them at highspeed, including internal insulating material of the electronicequipment, cushioning material and an insulating layer of the circuitboard.

1. A method of producing porous polyimide resin comprising: the resinsolution preparing process of preparing resin solution comprising aprecursor of polyimide resin, a dispersive compound capable ofdispersing in the precursor of polyimide resin, and a solvent; thecoating forming process of forming a coating in which the dispersivecompound is dispersed in the precursor of polyimide resin by applicationof the resin solution, followed by drying of the solvent; the extractingand removing process of extracting the dispersive compound from thecoating for removal by use of extracting solvent; the preheating processof preheating the coating in a temperature range of 190-250° C.; and theimidizing process of imidizing the coating.
 2. The method of producingporous polyimide resin according to claim 1, wherein the coating isheated in a temperature range of 270-400° C. in the imidizing process.3. The method of producing porous polyimide resin according to claim 1,wherein the dispersive compound is dispersed in the precursor ofpolyimide resin, with its average particle size of not more than 1 μm,in the coating forming process.
 4. The method of producing porouspolyimide resin according to claim 1, wherein the dispersive compoundhas a weight-average molecular weight of not more than 10,000.
 5. Themethod of producing porous polyimide resin according to claim 1, whereinthe extracting solvent used in the extracting and removing process isliquefied carbon dioxide or carbon dioxide that is in its super criticalstate.
 6. A porous polyimide resin which is produced in a producingmethod comprising: the resin solution preparing process of preparingresin solution comprising a precursor of polyimide resin, a dispersivecompound capable of dispersing in the precursor of polyimide resin, anda solvent; the coating forming process of forming a coating in which thedispersive compound is dispersed in the precursor of polyimide resin byapplication of the resin solution, followed by drying of the solvent;the extracting and removing process of extracting the dispersivecompound from the coating for removal by use of extracting solvent; thepreheating process of preheating the coating in a temperature range of190-250° C.; and the imidizing process of imidizing the coating, andwhich includes pores having an average pore size of not more than 1 μm.7. The porous polyimide resin according to claim 6, whose dielectricconstant is not more than 3.